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LABEX-G-EAU-THERMIE PROFONDE – Appel à projet 2015 – Projet soumis le 19 déc. 2014
Leveling and geodetic monitoring network around
Soultz-sous-Forêts and Rittershoffen geothermal sites
Gilbert Ferhat 1, 2, Jacques Ledig 2, Christine Heimlich 1
1: IPGS, UMR7516 CNRS/Univ. de Strasbourg, [email protected],
2: INSA de Strasbourg, [email protected]
Summary
We propose to investigate the surface deformation around the two geothermal exploitation sites at Soultz-sousForêts and Rittershoffen, by re-observing in 2015 the precision leveling network installed in May 2014 in the
frame of our former Labex project (Ferhat et Ledig, 2014). In May 2014, 43 leveling benchmarks were installed
more or less every km along the main roads surrounding and close the two geothermal exploitation sites. 13 out
of these 43 benchmarks are collocated with gravimetric sites (Hinderer et al. 2013, 2014) or continuous GNSS
antennas (Heimlich et al. 2013, 2014). This large network was entirely observed in May 2014 (about 40 km
long). A small loop of 3.3 km consisting of 5 leveling benchmarks surrounding the ECOGI site was measured
several times (May, June, July and November 2014) to monitor regularly the immediate vicinity of the newly
constructed ECOGI site. No vertical variations greater than 2 mm was observed around ECOGI between May
and November 2014.
Comparing 2014 and 2015 observations, we will be able to assess the stability of some area and also be able to
quantify/detect small vertical displacement of these leveling benchmarks. This information about vertical
displacements (or absence of movement) will be used to complement other geodetic measurements of
deformation performed since 2013 (GNSS, SAR observations, gravity measurements).
Moreover, we propose in 2015 to install a small precision leveling network in a suburb of Surbourg, which is
not covered by our 43 benchmarks, but is close to Soultz-sous-Forêts. Indeed, PS-INSAR observations in a
suburb of Surbourg during the period May 2012 and October 2014 revealed a mean rate of surface displacement
of -4 mm/yr in the Line of Sight (LOS) of the satellite (Heimlich et al., 2014). This leveling data will be helpful
in order to assess the deformation in this area and to deduce the horizontal part included in the LOS
displacement. Vertical observations will be linked to field observations in Surbourg showing small cracks in
some walls of houses. In addition, we plan to observe by GPS a few geodetic markers in Surbourg in order to
try to better understand such vertical and horizontal deformation processes.
In 2015, we also propose to repeat measurements by GPS campaign-mode of the positions of the geodetic
network used for the micro-gravimetric sites, sites observed by GPS/total station in October and November
2013.
We request financial support for field measurements and equipment.
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LABEX-G-EAU-THERMIE PROFONDE – Appel à projet 2015 – Projet soumis le 19 déc. 2014
1. Decription of the 2014 Labex project: precise leveling network
a. Different geodetic techniques for surface deformation monitoring
In order to monitor the surface deformation and deep mass redistribution around the two geothermal exploitation
sites, several geodetic measurements were performed in 2013 and 2014:
-
Repeated absolute and relative gravimetric measurements (Hinderer et al., 2013, 2014)
InSAR and continuous GPS measurements (Heimlich et al., 2013, 2014)
Leveling observations (this project, Ferhat & Ledig, 2014).
Figure 1: Map of the 6 permanent GNSS network (GPK1, GPK2, PYR4, ECOGI, Rittershoffen, Hatten and
Stundwiller) and the micro-gravimetric network of 13 sites (triangles) observed around the two geothermal sites
of Soulz-sous-Forêts and ECOGI (Rittershoffen) (NE of France).
Figure 1 shows the location of the 6 permanent GNSS antennas. It covers more or less the studied area. These 6
antennas were installed in 2014. The sites of main importance – i.e. exploitation sites – GPK1, GPK2 and
ECOGI are equipped with such antennas. Unfortunately, in April the GNSS receiver at PYR4 (also known as
OPS4 site) was stolen. GPK1, GPK2 and PYR4 are collocated with gravimetric measurements.
A micro-gravimetric network was observed in July/August 2013 and July/August 2014. It consists of 13 points
located around the Soultz-sous-Forêts and two points located around the ECOGI site (Hinderer et al., 2013). A
strong effort was done to define a homogenous network around the Soultz-sous-Forêts site. Due to the large
number of gravimetric loops and the size of the Soultz-sous-Forêts network, only two sites were observed around
ECOGI site (figure 1).
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LABEX-G-EAU-THERMIE PROFONDE – Appel à projet 2015 – Projet soumis le 19 déc. 2014
b. Large leveling network around Soultz-sous-Forêts and ECOGI sites.
To complement these geodetic measurements (cGPS and gravimetric measurements), leveling observations were
performed between these 13 gravimetric measurement sites. All gravimetric sites were equipped with a leveling
benchmark (Table 1). Due to the large distances, 3 GNSS were not included in our large leveling network
(Rittershoffen, Hatten and Stundwiller). GPK1, GPK2 and PYR4 sites are equipped with a leveling benchmark
(BM). No BM has been placed directly close the ECOGI GNSS site due to access problem and construction
zone.
Number
Continuous GNSS
measurements
cGPS-GPK1
Collocated Leveling benchmarks, BM
1
Gravimetric
sitename
GPK1
2
GPK2
cGPS-GPK2
3
PYR1
None
R15 close to the cGPS-GPK1 site
R15A close to the gravimetric site
R3
4
PYR2
None
R4
5
PYR3
None
R9
6
PYR4_SLTZ
< April 2014
R19
7
Bunker,
South of ECOGI
None
R36
8
Betschdorf
bunker
None
R32
9
Soultz Church
None
R2
10
Farm
None
R18
11
Chapel
None
R12
13
Hohwiller
None
R23
R5
Table 1 : Collocated sites by 2 or 3 techniques (cGPS, gravimetric measurements and leveling measurements) in
the Soultz-sous-Forêts and Rittershoffen area. Locations of the BM are indicated in figure 2.
For practical field measurements and leveling technical procedures, additional benchmarks were added between
the 13 gravimetric sites leading to a large leveling network of 40 km long and 43 leveling benchmarks (Figure
2). Depending on the location and topography, benchmark inter-distances vary from several meters to less than 2
km. In May 2014, these 43 leveling benchmarks were observed by 15 students at INSA during their field
campaign project. Using 4 benchmarks of the French Mapping Agency (R1, R8, R31 and R38, see Figure 2), we
were able to compute the elevation of all BM with an relative accuracy of 2 to 5 mm between consecutive
benchmarks (Ferhat et al. 2014b).
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LABEX-G-EAU-THERMIE PROFONDE – Appel à projet 2015 – Projet soumis le 19 déc. 2014
Figure 2: Leveling network around Soultz-sous-Forêts and Rittershoffen. Five loops were defined to perform
control of closure. Loop 5 surrounds the ECOGI site. Sites A, B and C show a zoom were several BM very
closed from each other exist (Ferhat et al. 2014b).
2. Results of our 2014 leveling measurements
a. Global leveling network
Table 2 summarizes the main results of data analysis. It is classical to compute a closure on a leveling loop to get
an idea of the precision of the measurements and to estimate a standard deviation of the error per km of leveling
line. With our procedure and instruments, the expected vertical precision announced by manufacturers are :
- 1 mm /km with standard rod
- 0.3 mm / km for highly precise leveling using invar rod.
Loop 1 to 4 were observed using digital levels (Leica DNA03) with standard rods, whereas Loop 5 was observed
with same levels but with invar rods. Only two invar rods were available at the time of the measurements.
Equivalent precision are acceptable (i.e. about 1 mm/km) even if loop 2 East exhibits 2.4 mm/km due may be to
steep slope in the topography.
Loop
Closure
(mm)
Number
of
stations
Loop 1
Loop 2 East
Loop 2 West
Loop 3
Loop 4
Loop 5
+12.2
+17.3
-6.5
+8.0
-11.7
+1.5
Total
145
158
85
126
129
46
689
Individual
elevation
difference (max
in mm)
1.0
6.7
1.2
1.8
1.2
1.8
6.7
Length
(km)
8.063
7.167
4.643
7.442
9.256
3.281
39.852
Equivalent
precision
(mm/km)
1.5
2.4
0.2
1.1
1.3
0.5
Balanced in sights
difference (m)
+9.7
+1.0
-2.6
+0.2
-2.7
+0.1
Table 2: Statistics for each loops of the 2014 observations in the studied area (Ferhat et al., 2014b). Loops 1 to 4
were observed in May 2014. Loop 5 were observed in June 5, 2014.
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LABEX-G-EAU-THERMIE PROFONDE – Appel à projet 2015 – Projet soumis le 19 déc. 2014
b. Loop 5 surrounding the ECOGI site
In order to monitor regularly the ECOGI area, several leveling campaigns of Loop 5 were performed in May,
June, July and November 2014. Loop 5 was observed entirely on June 5, June 18 and November 27. In July, corn
stems were to high to enable optical measurements in some locations. For these 3 dates, closure varies from 1 to
2 mm, leading to an equivalent precision of 0.2 to 0.5 mm/km. For most height differences changes are less than
1 or 2 mm (figure 3), except for the section R33-R34 where variation reach 5 mm at the period including
measurements performed in June 5, 2014. No clear explanation of such high rate could be given at that time.
Except for that anomalous variation, no vertical displacement higher than 1-2 mm could be observed during the
period May – November 2014 for the 5 benchmarks of the small loop 5.
Difference in heights (mm )
3
2
1
0
R33-R36
-1 0
-2
1
2
3
4
5
6
7
R36-R37
R37-R34
R34-R35
-3
-4
R35-R33
Months after May 2014
-5
-6
Figure 3: Variations of the height differences between consecutive leveling benchmarks of loop 5 observed in
four periods in 2014 : May 9–June 5, June 5–June 18, June 18–July 16 and July 16–November 27. See location
of the 5 benchmarks (R33, R34, R35, R36 and R37) on Figure 2. All data are referred to the first measurements
in May 2014.
d. Comparison with old IGN data (1973 and 1976)
Our studied area includes 5 leveling benchmarks installed by the IGN, the French Mapping Agency. These 5
leveling benchmarks are included in our global leveling network (R8, R10, R31, R37 and R38, see figure 2 for
location). Even though, the 2014 Labex Project did not finance the cost of old IGN data, through another project
we could get access to quite recent data (1973, 1976). By comparing observed height differences in 2014 and the
ones in 1973 or 1976, we obtain vertical rates varying from 0.1 to 0.6 mm/yr, much too close to their
uncertainties. No conclusions could be drawn at this stage from these old data.
3 Aims and objectives for the 2015 project
a. Main objective
The main objective of this proposal is to re-observe the main precision leveling network around the two sites of
Soultz-sous-Forêts and Rittershoffen. This leveling network will be measured regularly, at least once a year,
more frequently if anomalous deformations are detected, or depending of the injection program. More regular
surveys will be performed on Loop 5 (ECOGI site).
b. Second objective: leveling network and geodetic network in Surbourg area.
We propose in 2015 to install a small precision leveling network in a suburb of Surbourg which is not covered
by our 43 benchmarks, but is close to Soultz-sous-Forêts. Indeed, PS-INSAR observations in a suburb of
Surbourg during the period May 2012 and October 2014 revealed a mean rate of surface displacement of -4
mm/yr in the Line of Sight (LOS) of the satellite (figure 4, Heimlich et al., 2014). This leveling data will be
helpful in order to assert the deformation in this area and to deduce the horizontal part included in the LOS
displacement. Vertical observations will be linked to field observations in Surbourg showing small cracks in
some walls of houses. Attempt to explain such deformation should be done.
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LABEX-G-EAU-THERMIE PROFONDE – Appel à projet 2015 – Projet soumis le 19 déc. 2014
Figure 4 : Mean velocity map obtained form PS-INSAR date (May 2012 – October 2014) (Heimlich et al. 2014).
This study will be profitable for the analysis of the origin of the displacement in Surbourg: is it related to a near
surface origin (e. g. compaction of clay) or is it related to aseismic displacement on deep fault? The pattern of
displacement is the orientation of a fault close to this location.
More precisely, we propose a coverage of the area with profiles along the streets of Surbourg, in the East part : 2
East-West profiles and 2 North-South profiles connecting together. The reference point will be close to the
village hall. This point is in the continuity of a point which is already measured for the gravity survey. The
connection of this new survey and the existing survey will than be possible and interesting to observe.
Figure 5: Proposed leveling lines in the suburb of Surbourg, South of Soultz-sous-Forêts. Proposed geodetic
network to be observed by GPS is not shown on the figure.
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LABEX-G-EAU-THERMIE PROFONDE – Appel à projet 2015 – Projet soumis le 19 déc. 2014
c. third objective
A third objective is to re-survey the existing micro-gravimetric network (Figure 2). A micro-gravimetric network
was observed in July/August 2013 and 2014. It consists of 13 points located around the Soultz-sous-Forêts and
two points located around the ECOGI site (Hinderer et al., 2013, figure 1). A strong effort was done to define a
homogenous network around the Soultz-sous-Forêts site. Due to the large number of gravimetric loops and the
size of the Soultz network, only two sites were observed around ECOGI sites.
These 13 points were tied the national reference system using GNSS and total station measurement (Ferhat,
2013). We could achieve for some points a precision of 3-6 mm in horizontal and 5-9 mm in vertical component
(Collignon et Hagenmuller, 2014).
4. Leveling surveys as a complementary tool for monitoring surface deformation
Repeated leveling surveys along specific zones (i.e. ECOGI monitoring site), or in the entire studied area and the
re-measurement of the geodetic of the micro-gravimetric network should:
-
enable us to estimate the precise altitude of ECOGI monitoring network (a few mm)
-
enable control of deformation rates obtained by SAR and permanent GPS (Heimlich et al., 2013, 2014)
for the 13 sites and especially for the Surbourg area.
-
lead us to evaluate the vertical deformation on the micro-gravimetric network. This information is
crucial for correctly interpret gravity variations. It is necessary to be able to dissociate motion of the
ground (uplift, subsidence) with the redistributions of mass at depth linked to hydrological or
geothermal processes.
4. Field measurements in 2015
They would consist of different actions:
-
-
Reconnaissance for sites to be leveled and installation of several leveling and geodetic benchmarks in
the Surbourg area.
Observations by Team 1 of a local leveling network surrounding the ECOGI sites (3.3 km long) and
Surbourg area. Team 1 comprises 2 trainees and G. Ferhat. C. Heimlich will participate in the choice of
selected area in Surbourg and will perform GPS measurements in Surbourg.
Observations by Teams 1 and 2 of a main leveling network from Soultz-sous-Forêts to ECOGI site
(about 40 km long). Team 2 is a group of students from INSA de Strasbourg.
Observations by GPS and total station of the geodetic network used for micro-gravimetric
measurements (Team 1) and also in Surbourg.
Students have applied already for the fieldwork in June and July 2015 (Team 1). In 2014, the large leveling
network (40 km) could be observed thanks to the participation of a large group of fifteen surveyor students at
INSA. The 2015 survey will be scheduled again in the same period (i.e. May).
5. Synergy with work packages of the main Labex proposal
This work contributes to WP3 and WP5 of the main proposal:
- it will provide information on the surface deformation above the geothermal exploitation (WP3)
- it will involve many students as trainees or surveyor students as part of their education in field
measurement (“Projet groupé du mois de mai”) (WP5)
Our proposal have some interactions with two previous 2013 and 2014 projects:
- A geodetic study of the Soultz-sous-Forêts and Ritteshoffen geothermal sites (F. Masson)
- Preliminary study of the monitoring of a geothermal reservoir by hybrid gravimetry (J. Hinderer)
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LABEX-G-EAU-THERMIE PROFONDE – Appel à projet 2015 – Projet soumis le 19 déc. 2014
Results of our 2014 project were presented at two conferences:
-
a poster at the European Geothermal Workshop in Karlsruhe (Ferhat et al., 2014a)
a talk at the European Geothermal Workshop in Karlsruhe (Hinderer et al., 2014)
a talk at Colloque de Géodésie et Géophysique G2 in Strasbourg (Ferhat et al., 2014b)
6. Budget
The budget is composed of equipments and fieldwork. The 2014 experience shows that lack of a high precise
level limits field measurements as well as the lack of invar rods, because the INSA instruments were not always
available for measurements. This is why, we request for these necessary equipments.
Because the processing of the data is done with Excel computations sheets, we request for a commercial
software in order to gain time in the processing of the data.
Fieldwork are quite heavy because of the size of the network (40 km), this could be done in 2014 with the
participation of 15 students. Again, in 2015 a large group of students will be involved in the field measurements.
Equipments
-
High precision level with two rods:
Quotation provided by Geotopo/Trimble.
- Acquisition of supplementary invar rods.
- Acquisition of adjustment software (GEOLAB)
6 700 € HT
2 000 € HT
2 500 € HT
Fieldwork
Team 1 (March to June/July 2015) GPS and leveling:
4 weeks (20 days) x 4 persons (3 students + G. Ferhat):
Transportation (150 km/day x 20 days x 0.30 euros/km):
Team 2 (May 2015) leveling only:
5 days x 15 surveyor students x 1 technician + J. Ledig
Transportation and small material for monumentation of the
benchmarks are included (5 days x 700 € /day)
Total:
Left 3400 € HT from 2014 project
1 800 € HT
900 € HT
3 500 € HT
17 400 € HT
14 000 € HT
7. References
Collignon A. et V. Hagenmuller, 2014, Travaux topographiques en vue de l’auscultation topographique
du site de géothermie de Soultz-sous-Forêts et des sites alentours, Rapport de PRT, Projet de Recherche
Technologique sous la direction de G. Ferhat, Spécialité Topographie de l’INSA de Strasbourg.
Ferhat, G., 2013, Rattachement géodésique des sites de gravimétrie relative pour le suivi temporel des
réservoirs géothermiques de Soultz-sous-Forêts et Rittershoffen (ECOGI), Lettre de l’EOST, 16, page 8.
Ferhat G., G. Ménard, J.-P. Malet & J. Hinderer, 2014a, Vertical ground deformation monitored by
precise leveling: case study of Upper Rhine Graben, geothermal exploitations sites Soultz/Rittershoffen, Alsace
and Ubaye, Southern Alps. Colloque de Géodésie-Géophysique G2, Strasbourg, France, 17-19 November 2014.
Ferhat G., V. Patoine & E. Clédat, 2014b, Leveling network for surface deformation monitoring along
Soultz-sous-Forêts and Rittershoffen geothermal sites, France. European Geothermal Workshop, Karlsruhe,
Germany, 15-16 October 2014.
Heimlich C., F. Masson & N. Gourmelen, 2013, Geodetic monitoring strategy at the geothermal sites of
Soultz-sous-Forêts and Rittershoffen (Upper Rhine Graben, France), 2nd European Geothermal Workshop,
Strasbourg, France
Hinderer J., B. Hector, U. Riccardi, G. Ferhat, & J.-D. Bernard, 2013, Preliminary results of the
monitoring of the Soults-sous-Forêts geothermal reservoir by hybrid gravimetry, 2nd European Geothermal
Workshop, Strasbourg, France.
Hinderer J., B. Hector, M. Calvo, U. Riccardi, G. Ferhat, Y. Abdelfettah & J.-D. Bernard, 2014,
Monitoring of geothermal reservoirs by hybrid gravimetry. European Geothermal Workshop, Karlsruhe,
Germany, 15-16 October 2014.
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